The present invention relates to methods and apparatus for reviving an electrode of a biosensor, and more particularly to methods and apparatus for reviving an electrode of a biosensor by supplying a bias having a different polarity from the measurement bias polarity to the electrode before a measuring operation is started.
It is known that a physiologically active substance is capable of selectively detecting a very complicated organic compound, protein or the like with high sensitivity. With attention directed to this characteristic, research has been made in various biosensors.
A typical biosensor having an electrode unit and a physiologically active substance fixed thereon is proposed. The biosensor is used for detecting the existence of an object substance, the relative or active quantity of the object substance and the like based on an electrical signal output from the electrode unit corresponding to the biological reaction of the object substance, under the condition that a predetermined forward bias is applied to the electrode unit. For example, the electrode unit has a working electrode made of platinum and a counter electrode made of silver. An enzyme-immobilized membrane is fixed onto the electrode unit. When the object substance is to be measured, hydrogen peroxide is generated through an enzyme reaction of the object substance. Then, the generated hydrogen peroxide reaches the surface of the electrode unit through a hydrogen peroxide penetration membrane. The electrode unit outputs an electrical signal corresponding to the quantity of the hydrogen peroxide that reaches it. The existence of the object substance, the relative or active quantity of the object substance and the like are detected based on the electrical signal. Concretely, the working electrode has a forward bias of 0.6 volts applied with respect to the counter electrode.
In the biosensor described above, an interfering layer such as an oxidized layer and the like which interferes with electrical signals is generated on the working electrode, and the activity of the working electrode is lowered as the object substance measuring operation is continued with the forward bias being applied to the electrode unit. It is proposed that a reverse bias is accordingly applied to the electrode unit (the working electrode has a reverse bias of about -0.6 volts applied with respect to the counter electrode, corresponding to the concrete example) after one or more measurements are performed, within the time period in which the measurement is not performed, so as to remove the interfering layer. Then the activity of the working electrode is revived. The level of the output signal from the electrode unit is raised to the original level. (Refer to Japanese Patent Laid Open Sho 60-155959).
Lowered measuring sensitivity is recovered by applying the predetermined reverse bias voltage to the electrode unit during the time period in which the measurement is not performed, as is described above, then measuring with high sensitivity can be performed again.
In the reviving apparatus described above, a constant-voltage for reviving (reverse bias) is applied between the working electrode and the counter electrode. The oxidized layer on the working electrode is reduced, hydrogen, hydrogen ions and the like are accordingly generated. When a constant-voltage for measurement (forward bias) is applied between the working electrode and the counter electrode, a diffusion current flows depending upon the generated hydrogen, hydrogen ions and the like.
As a result, the electrical signal provided for measuring is affected by being lowered, by the diffusion current, so as to remarkably lower measuring accuracy just after the constant-voltage for measurement is applied between the working electrode and the counter electrode. When the measurement is performed after the diffusion current is sufficiently lowered, sufficient measuring accuracy is obtained, but a remarkably long waiting time period (for example, 1-2 minutes) is needed.
The same disadvantage as above arises for a biosensor having a reference electrode in addition to the working electrode and the counter electrode.
Also, in the reviving apparatus described above, reverse bias for reviving is applied between the working electrode and the counter electrode (refer to FIG. 13(A)) every predetermined time (for example, every hour) and just before starting of measurement. When the reverse bias for reviving is applied between the working electrode and the counter electrode, a large current flows in thin membranes fixed on a surface of an electrode unit (refer to FIG. 13(B)). Thin membranes, especially a selective penetration membrane being positioned close to the electrode unit, are gradually destroyed, thereby the penetration quantity of substances generated by the reaction or substances consumed by the reaction increases. For example, when the biosensor is used to measure glucose concentration, the hydrogen peroxide selective penetration membrane, made of a cellulose acetate membrane is destroyed, thereby the quantity of penetrating hydrogen peroxide increases. As a result, there is a disadvantage that an electrical signal provided from the electrode unit increases depending on the number of times a reviving operation occurs. Another disadvantage is that the life of the membrane or membranes is shortened by up to about one month. The life is a time period until a penetration ratio of the hydrogen peroxide selective penetration membrane increases by 10%.